Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
Background Art
Power integrated circuits (power ICs) that include both a vertical power semiconductor device and a lateral power semiconductor device for controlling/providing a protection circuit for the vertical power semiconductor device are a well-known conventional technology for increasing the reliability and reducing the size and cost of power semiconductor devices (see Patent Documents 1 and 2, for example). One example of such a conventional power semiconductor device is a power IC that includes an output stage vertical power semiconductor device, a circuit device for a control circuit, and a protection device that are all mounted on the same semiconductor substrate. FIG. 11 is a cross-sectional view illustrating the structure of an example of such a conventional semiconductor device.
The conventional semiconductor device illustrated in FIG. 11 is an example of a high-side power IC for use in a vehicle and includes an output stage vertical power semiconductor device arranged on a vertical trench-gate metal-oxide-semiconductor field-effect transistor (MOSFET) 110. The power IC also includes a lateral p-channel MOSFET and a lateral n-channel MOSFET connected together complementarily to form a lateral complementary MOS (CMOS) that functions as a circuit device for a control circuit. However, FIG. 11 only depicts the lateral n-channel MOSFET 120.
As illustrated in FIG. 11, the conventional semiconductor device includes a single semiconductor substrate (semiconductor chip) 100, which is divided into an output stage portion in which the output stage vertical power semiconductor device is arranged and a circuit portion in which components such as the circuit device for the control circuit and the protection device are arranged. The semiconductor substrate 100 is formed by epitaxially growing an n− semiconductor layer 102 on the front surface of an n+ supporting substrate 101. The output stage vertical MOSFET 110 is formed in the output stage portion. In the output stage portion, the n+ supporting substrate 101 and the n− semiconductor layer 102 respectively function as an n+ drain layer and an n− drift layer. A drain electrode 109 (a drain terminal) is connected to the rear surface of the supporting substrate 100 (that is, to the rear surface of the n+ supporting substrate 101) and functions as a supply voltage terminal (hereinafter, “VCC terminal”) that is connected to a vehicle battery.
A ground terminal (hereinafter “GND terminal”) and an output terminal (hereinafter, “OUT terminal”) are formed on the front surface side of the semiconductor substrate 100 (that is, on the side of the n− semiconductor layer 102 opposite to the n+ supporting substrate 101 side). The OUT terminal is electrically connected to an n+ source region 107 and a p++ diffusion region 108 of the vertical MOSFET 110. The vertical MOSFET 110 also includes a trench 103, a gate insulating film 104, a gate electrode 105, and a p-type base region 106. In the circuit portion, elements such as a lateral CMOS for the control circuit and a diffusion diode 130 are formed. A lateral n-channel MOSFET 120 that is part of the lateral CMOS in the circuit portion is arranged within a p-type base region 121 that is selectively formed in the surface layer of the front surface of the substrate.
The diffusion diode 130 is a lateral diode that includes a p-type diffusion region (hereinafter “p-type anode region”) 131 that is selectively formed in the surface layer of the front surface of the semiconductor substrate 100 and separated from the p-type base region 121. The diffusion diode 130 functions as a protection device for protecting (providing gate protection for) a gate insulating film 124 of the lateral n-channel MOSFET 120 that is part of the lateral CMOS for the control circuit. A cathode terminal is connected to an n+ cathode region 132 of the diffusion diode 130 as well as to a gate electrode (gate terminal) 125 of the lateral n-channel MOSFET 120. Moreover, an anode terminal is connected to a p+ anode contact region 133 of the diffusion diode 130 as well as to the GND terminal. The lateral n-channel MOSFET 120 also includes an n+ source region 122 and an n+ drain region 123.
In the power IC illustrated in FIG. 11, the diffusion diode 130 undergoes breakdown (Zener breakdown), and the breakdown voltage of the diffusion diode 130 is applied to the gate terminal VG of the lateral n-channel MOSFET 120. Furthermore, the breakdown voltage of the diffusion diode 130 is also applied to the gate terminal of the lateral n-channel MOSFET 120 even when a high input voltage (that is, the output control signal of the lateral n-channel MOSFET 120) is used, and therefore a high voltage is not applied to the gate insulating film 124 of the lateral n-channel MOSFET 120. In other words, the diffusion diode 130 clamps (fixes) the input voltage applied to the gate terminal VG of the lateral n-channel MOSFET 120 to a prescribed voltage, thereby also protecting the gate insulating film 124 of the lateral n-channel MOSFET 120.
Meanwhile, if the gate insulating film 124 of the lateral n-channel MOSFET 120 is formed more thickly, the operating voltage (gate voltage) for driving the lateral n-channel MOSFET 120 must be increased. In this case, if the breakdown voltage of the diffusion diode 130 is too low, a sufficiently large gate voltage will not be applied to the lateral n-channel MOSFET 120 even if a high input voltage is used. As a result, a sufficiently large current does not flow between the source and drain of the lateral n-channel MOSFET 120, and the power IC may function abnormally. Therefore, if the breakdown voltage of the diffusion diode 130 is too low when only a single diffusion diode 130 is used, a plurality of diffusion diodes 130 are connected in series to form a multi-stage diode. FIG. 12 is a cross-sectional view illustrating the structure of another example of a conventional lateral protection device.
FIG. 12 illustrates an example of the cross-sectional structure of a multi-stage diffusion diode 130 that includes three stages 130a to 130c arranged in order from the upstream side (that is, from the lateral n-channel MOSFET 120 side (the upper side in the figure)). As illustrated in FIG. 12, connecting together the plurality of diffusion diodes 130a to 130c to form a lateral protection device that provides gate protection for the lateral n-channel MOSFET 120 increases the magnitude of the voltage to which the lateral protection device clamps the input voltage, thereby ensuring that a sufficient gate voltage is applied to the lateral n-channel MOSFET 120. FIG. 12 depicts a case in which the plurality of connected diffusion diodes 130a to 130c are reverse-biased such that a first terminal 136 of the lateral protection device has a higher voltage than a second terminal 137.
As illustrated in FIG. 12, the diffusion diodes 130a to 130c are respectively formed in p-type anode regions 131a to 131c, which are formed separated from one another in the surface layer of the front surface of the semiconductor substrate 100. The lateral protection device also includes n+ cathode regions 132, p+ anode contact regions 133, anode electrodes (anode terminals) 134, and cathode electrodes (cathode terminals) 135. The letters “a” to “c” are appended to the reference characters of these components to indicate which components correspond to the respective diffusion diodes 130a to 130c. The anode electrodes 134a and 134b of the diffusion diodes 130a and 130b are respectively connected to the cathode electrodes 135b and 135c of the downstream diffusion diodes 130b and 130c. In other words, the diffusion diodes 130a to 130c are formed by the p-n junctions in diffusion regions, and the diffusion diodes 130a to 130c are connected together in series.
The cathode electrode 135a of the diffusion diode 130a and the anode electrode 134c of the diffusion diode 130c function as connection points with other components (that is, as the first and second terminals 136 and 137). FIG. 13 illustrates an example of a power IC in which the plurality of connected diffusion diodes 130a to 130c are used as a lateral protection device 141 that provides gate protection for the lateral n-channel MOSFET 120. FIG. 13 is a circuit diagram of a power IC that includes the protection device illustrated in FIG. 12. As illustrated in FIG. 13, the first terminal (cathode terminal) 136 of the lateral protection device is connected to the gate terminal VG of the lateral n-channel MOSFET 120, and the second terminal (anode terminal) 137 of the lateral protection device 141 is connected to the source terminal of the lateral n-channel MOSFET 120. If the gate insulating film of the lateral n-channel MOSFET 120 is formed thickly, a relatively high gate voltage must be applied to the lateral n-channel MOSFET 120 in order to ensure that a sufficient current flows between the source and drain thereof.
Next, the operation of the conventional power IC illustrated in FIG. 13 will be described. The lateral n-channel MOSFET 120 is used in a state output circuit (a diagnosis circuit) or the like, for example, and turns ON and OFF to control the power IC. An output control signal that turns the lateral n-channel MOSFET 120 ON and OFF is input to a first input terminal 144. When the output control signal is in the Low state, the lateral n-channel MOSFET 120 remains in the OFF state. However, when the output control signal is in the Hi state, the lateral n-channel MOSFET 120 is switched ON and current flows through the lateral n-channel MOSFET 120 to outside of the power IC. The output control signal is a high voltage and is decreased using a resistor 142 and the lateral protection device 141. The lateral protection device 141 is reverse-biased and undergoes breakdown, thereby clamping the gate voltage applied to the lateral n-channel MOSFET 120. Therefore, a sufficient voltage that is equal to the breakdown voltage of the lateral protection device 141 is applied to the gate terminal VG of the lateral n-channel MOSFET 120.
Moreover, an output interrupting n-channel MOSFET 143 is arranged between the resistor 142 and the first terminal 136 of the lateral protection device 141. When the lateral n-channel MOSFET 120 is outputting current, the n-channel MOSFET 143 interrupts that output current according to an interrupt control signal sent from an interrupt signal circuit (not illustrated in the figure). For example, the interrupt control signal input to a second input terminal 145 is set to the Hi state when another protection circuit (not illustrated in the figure) detects that the power IC is in an abnormal state, thereby switching ON the n-channel MOSFET 143. This pulls down the voltage applied to the gate of the lateral n-channel MOSFET 120, thereby switching OFF the lateral n-channel MOSFET 120. In this type of power IC, the breakdown voltage of the lateral protection device 141 must be set to a value that is greater than or equal to a threshold voltage sufficient to switch ON the lateral n-channel MOSFET 120 but less than the breakdown voltage of the gate insulating film.
Patent Document 3, for example, discloses a device for use as the diffusion diode in this type of power IC. The device includes a p-type semiconductor region formed beneath a cathode region and an n+ embedded layer formed beneath the p-type semiconductor region, thereby reducing the severity of the parasitic transistor effect that occurs directly beneath the cathode region.